1. Field of the Invention
This invention relates to a corona discharging device, and more particularly to a corona discharging device for uniformly discharging or charging the surface of an image bearing member such as a photosensitive member or an insulating member.
2. Description of the Prior Art
As an image bearing member in the ordinary electrophotographic method, there is known, in addition to a photosensitive member, an insulating member used to form a secondary latent image by an ion flow and by utilization of a latent image formed on the photosensitive member. These image bearing members are sometimes in the form of a drum or sometimes in the form of an endless web. Generally, an electrostatic latent image is formed on such image bearing member and this latent image is developed with the aid of a developer containing toner, whereafter the developed image is transferred to a transfer medium. After the image transfer, the image bearing member is cleaned by cleaning means to remove any developer remaining on the image bearing member, thus becoming ready for another cycle of image formation.
The conventional photosensitive medium can form thereon a latent image having a high electrostatic contrast of about 350-500 volts and therefore, the unevenness of some charges remaining on the photosensitive medium offers little problem. However, where the charge retaining capability of the surface of the photosensitive medium is low due to the characteristic of the surface layer or an electrostatic latent image having a high potential cannot be formed due to a special latent image formation process, if the uneven potential remaining on the photosensitive medium after the image transfer is not uniformized, it will undesirably present itself as irregularity. Particularly, an electrostatic latent image obtained by simply transferring a latent image formed on a photosensitive medium onto a latent image drum or by a process in which a secondary electrostatic latent image is formed on an insulating drum by modulating an ion flow by a primary electrostatic latent image formed on a screen-like photosensitive medium (hereinafter referred to as the screen) sometimes has an electrostatic contrast lower than 150 volts, so that the visible image resulting therefrom is disturbed by the above-described uneven potential. What is considered to be the cause of the uneven potential created on the image bearing member after the image transfer includes irregular developing effect, application of a bias voltage during the image transfer, and residue of the transferred image.
When the electrostatic contrast of the latent image formed on the image bearing member is low as described above and the image member is repetitively used, it is necessary to render the entire surface of the image bearing member to a specific uniform level.
As the means for such uniformization (usually, discharging) of the image bearing member surface, there is known the AC corona discharger. However, in the AC corona discharging, only the corona discharging action of one polarity is effectively utilized and so, a long time is required for discharging, and this cannot be said to be so suited for high-speed image formation apparatuses.
As another method, Japanese Patent Publication No. 23181/1976 discloses an example in which a voltage of the opposite polarity (for example, negative) to the residual charges is applied to a corona discharger to change an image bearing member to the saturated charge potential thereof, whereafter positive corona discharge is uniformly imparted by changing the polarity of the applied voltage to thereby render the surface potential to a desired value.
However, image bearing members which can be charged to the saturated charge potential thereof are limited and in most cases, partial dielectric breakdown occurs before the uniform saturated charge potential is reached and thus, not only potential irregularity occurs but also the image bearing member itself is often broken down. Moreover, in the above-mentioned example, it is difficult to control the final surface potential of the image bearing member always to a stable value and an inconvenience has been encountered that when images are repetitively formed, differences occur in density of the images.
Therefore, in U.S. Application Ser. No. 884,242 previously proposed by the inventor of the present application, the surface of the image bearing member is first greatly charged by positive corona, and then a negative corona discharging device having a grid is operated to uniformize and stabilize the final surface potential of the image bearing member. Also, simultaneously with the uniformization of the potential of the image bearing member, toner is charged to the same polarity as the polarity applied to the screen to thereby prevent the screen from being stained.
In this example, however, since the image bearing member is greatly charged by positive corona discharge and then the negative corona discharging device having a grid is operated to render the surface of the image bearing member to a uniform negative potential, a great deal of negative corona current flows at this time. Particularly, if the grid is provided in the discharging opening portion, much current flows through the grid to increase the overall current. Thus, particularly, ozone or other harmful substances may be created to harm the human bodies or deteriorate the insulation characteristic of the insulating layer and/or the photoconductive layer, or oxides formed in the air by ozone may undesirably corrode the metals.